This is an array struct template, this code compiles:

struct Vec(size_t N) {
    void opBinary(string op:"~", size_t M)(int[M]) {}
}
void main() {
    Vec!2 a1;
    int[3] a2;
    a1 ~ a2; // OK
}



But this code doesn't compile:

struct Vec(size_t N) {
    void opBinary(string op:"~", size_t M)(Vec!M) {}
}
void main() {
    Vec!2 a1;
    Vec!3 a2;
    a1 ~ a2; // line 7, Error
}


DMD 2.049 shows:
test.d(7): Error: cannot implicitly convert expression (a2) of type Vec!(3) to Vec!(M)

Can you show me how to fix the second program?

Bye and thank you,
bearophile

bearophile <bearophileHUGS@lycos.com> wrote:

> Can you show me how to fix the second program?

This is issue #3467 - Non-int integral template parameters not correctly propagated

The solution is to use int instead of size_t:

struct Vec(int N) {
    void opBinary(string op:"~", int M)(Vec!M) {}
}
void main() {
    Vec!2 a1;
    Vec!3 a2;
    a1 ~ a2; // line 7, Error
}

-- 
Simen

This version works, but it requires a hidden field with distinct name (and no gensym at compile time is available). Do you know a better solution that avoids the need for the distinct hidden field?


struct Vec(size_t N) {
    enum size_t _hiddenVecLength = N;
    void opBinary(string op:"~", T)(T) if (IsVec!T) {}
}
struct Fake(size_t N) {
    enum size_t _hiddenVecLength = N;
}
template IsVec(T) {
    enum bool IsVec = is(typeof(T._hiddenVecLength) == size_t)
                      /* && is(T == Vec!(T._hiddenVecLength)) */ ;
}
void main() {
    Vec!2 a1;
    Vec!3 a2;
    a1 ~ a2;
    Fake!3 f;
    a1 ~ f; // bug
}

Bye,
bearophile

Simen kjaeraas:
> This is issue #3467 - Non-int integral template parameters not correctly
> propagated
> The solution is to use int instead of size_t:

Thank you! :-)

I'll add a template constraint to be sure M it's positive:
void opBinary(string op:"~", int M)(Vec!M) if (M >= 0) {}

Bye,
bearophile